Monitoring system



. Nov. 1, 1932. F. w. SCHRAMM MONITORING SYSTEM Filed Dec e, 1930 Osdllatar l D I Oscillato 2 Demodulatw G INVENTOR I Sc/wczum/m/ BY ATTORNEY Patented Nov. 1, 1932 reassur s PATENT o Ics w. soirn nran, or EAST ORANGE, new .rnRsnY; ASSIGNOR TO AMERICAN ram 'rrionn AND TELEGRAPH COMPANY, A oonronarron or new YORK,

MONITORING sysirnnr Application filed December 6, 1930, Serial No. 500,505.

This invention relates to -signaling systems.

I More particularly this invention relates to apparatus andmethods for monitoring said systems. 1 p W In radio receivers of, for example, the

double detection type, the intermediate frequency amplifying apparatus generally in cludes, in addition to one or more amplifying elements, a band-pass filter or its equivalent which transmits only a certain, well defined band of frequencies. The incoming sig nals which may be in the form of a carrier Wave and both side bands, are modulated with current of a beating oscillator in order to producecurrents corresponding to the carrier'and its side bands lowered in the fre quency spectrum and lying within the range of the band-pass filter. If variations occur either inthe frequency ofthe incoming carrier wave or in the frequency of the beating oscillator, the lntermediate frequencies Wlll be thrown wholly or partlyoutside the transmission band of the filter, causing greater attenuation of some frequencies than of others, thereby reducing the intelligibility of the signals. 7 V f When it is desired to determine the stability of the frequency ofthe incoming carrier waveor of the frequency of the oscillator used for beating purposes in the process of modulation, the intermediate frequency corresponding to the carrier wave and resulting from modulation maybe adjusted to some predetermined point lying within the limits of the'transmisslon band of the filter by obtaining a zero beat w1th the current of another local oscillator fixed at this frequency.

l/l hile this adjustment is being made, however, the beat note Wlll appear in the output of .the receiver and the receiver must be temporarily, removed from service in; order to Tously monitoring the various frequencies without any interference from a beat note.

This invention will bebetter understood from the detailed description hereinafter 'following, when read in connection with the ac-i companying drawing showing one embodiment of the'invention. merely for the purpose ofillustration. 7 v j Referring to the drawing, the reference] character N represents an antenna upon which may be impressed a plurality of carrierwaves and corresponding side bands representing different speech or other signaling currents, only one of the carriers and its corresponding side bands-being desired. The desired carrier and its side bands may, of course, be selected in any way well known in the art. The selected carrier wave and its side bands {will be transmitted through an amplifier A iamplified thereby and then transmitted through a 'transformer T to the input circuit of a modulator designated M.

The selected and amplified carrier wave and its s de bands reach the'grid and filament electrodes of the tube M through a condenser C The output' circuit of this modulator includes the secondary winding of a transformerT the primary'winding of'a trans former T5, a condenser C and the plate and filament. electrodes of the tube M. An oscillator designated 0 is connected to the pri mary winding of the transformer T andsupplies current of the beating frequencyto be employed in the process of modulat on whereby 'theselected carrier wave and its side bands will be lowered in the frequency spectrum to the range of the filterF. Positive potential for the plate electrode of the tube 'M is supplied by a battery 13 through a circuit which includes the primary winding of transformer T the secondary winding'of transformer T pthe plate and filament-electrodes of the tube M, a resistance R 'and ground. It will be shown hereinafter how the drop in voltage across the resistance R provides part of the negative bias for the grid electrode of the tube M.

The carrier wave and its side bands lowered in the spectrum to intermediate fre- V, quencies as a result of modulation are transmitted through a band-pass filter F and then ampl fied by an amplifier designated A fixed frequency limits which may be spaced The filteris of any wellknown type, having by, for example, twice the width of the ordinary speech band. This filter will freely transmit the carrier wave and its side bands lowered in the spectrum through the process of modulation and all frequencies outside of its range will be greatly suppressed.

After amplification by the amplifier A the currents of the intermediate frequencies are then transmitted through a transformer T and a condenser C to the grid and filament electrodes of a detector designated D. The detector D further reduces the frequencies of the currents impressed upon its gridand filament electrodes to such low values as may be rendered audible by such devices as loud speakers. The detected currents are bypassed by a condenser C and are transmitted through a transformer T to an output circuit des gnated K. Any form of loud speaker may, for example, be connected or coupled to the output circuit K.

It is to be noted that a battery B supplies the necessary positive opera-ting potential for the plate electrode of the tube D, the completed circuit including battery B the primary winding of transformer T the plate.

and filament electrodes of the tube D, a resistance R and ground. The resistance R is also connected in series with the grid and filament electrodes of the tube D, the completed circuit including resistance R a resistance R the secondary winding of transformer T .and the grid and filament electrodes of the tube D. The drop in voltage across the resistance R supplies the negative bias for the grid electrode of the tube D.

The intermediate frequency potentials reach ng the grid and filament electrodes of the tube D are also impressed between the grid and filament electrodes of an amplifying tube V, the grids of tubes D and V being separated by a condenser C Condenser C prevents such direct currents as may flow through any of the circuits of the tube D from reaching the grid electrode of the tube V. The grid of the tube V may be connected to its grounded filament through a resistance R All of the intermediate frequency currents amplified by the tube V will flow through its output circuit which includes a condenser C the primary winding of a transformer T and the plate and filament elec trodes of the tube V. The positive operating potential forthe plate electrode of the tube V is supplied by battery B The secondary winding of the transformer T is connected in a circuit which includes resistances R and R a choke coil L a resistance R and the plate and filament electrodes of a rectifying tube E. By virtue of the rectifying action of the tube E there results a direct current which will produce a drop in potential across the resistance R and this potential drop will vary with the amplitude of the carrier wave as it reaches the tube D at mary winding of a transformer T a reduced frequency. As the amplitude of this wave increases, the potential drop across the resistance R will be increased, and conversely, as the amplitude of this wave decreases, the potential drop across the resist- .v

ance R, will be decreased.

It is to be noted that the filament of the tube E is grounded and that the resistance R is connected in a series circuit which includes a resistance R the secondary winding of transformer T the grid and filament electrodes of tube M, resistance R and ground. It will be apparent that any change in the potential drop across the resistance R will correspondingly vary the negative bias of the grid electrode of the tube M. As the voltage across the resistance R increases, the grid electrode of the tube M will become more negative and correspondingly reduce the gain of the tube M. Moreover, any reduction in the voltage drop across the resistance R will increase the gain of the tube M.

The winding L is of the iron core type and it tends to prevent alternating currents from flowing through resistance R and through any part of the rectifying apparatus includmg the tube E. Moreover, a condenser C freely transmits all alternating currents impressed upon the secondary winding of the transformer T to the circuit including the resistances R and R Of course, the condenser C also tends to prevent any alternating currents from flowing through the rectifying circuit formed in part by the tube E.

The resistances R and R form two of the arms of a Nheatstone bridge. Another of the arms of this bridge includes a Winding L of adjustable inductance and a condenser C of adjustable capacity. The fourth arm of the bridge includes a condenser C and the pri- This bridge may be balanced at any one frequency merely by adjustments of the reactive values of the winding L and condenser C The intermediate frequency currents passed by con- 1 denser C reach the diagonal of the bridge designated by the point a and b. An oscillator O is connected to the other diagonal of thebridge determined by the points 0 and d.

The secondary winding of the transformer T is connected to a demodulator G, in the output circuit of which there may be any form of indicating device such as a telephone receiver S.

The frequency of the oscillator O is adjustable as is the frequency of the oscillator 0 The frequency of the oscillator 0 may be fixed at any point which may lie within the limits determined by the filter F, this fre quency being that to which it is desired to adjust the frequency of the intermediate carrier wave. If the VVheatstone bridge circuit is balanced for the frequency of the oscillator no current of this frequency will be impressed upon the diagonal determined by the I ployed for the control of the amplification of the tube M. Inasmuch as current of the frequency' ofthe oscillator cannot reach the input circuit of the tube M, that current will.

not be modulated with the currents transmitted by the amplifier A and, of course,

no heat notes will be produced and transmitted to the output circuit K.

The frequency of; theicarrier-wave lowered by modulation does reach the diagonal of the lVheatstone bridge determined by the points a and b. If this frequency is different from that of the oscillator 0 both frequencies will be transmitted through the transformer T to the demodulator G. A beat note will then be heard in the telephone receiver S. The operator may then adjust the frequency of the oscillator O until the intermediate frequency of the carrier wave becomes equal to that of the frequency of the oscillator 0 When that occurs, no sound will be heard at the telephone receiver S.

It will be apparent that the monitoring apparatus is continually in operation and that the carrier frequency is continually monitored, and any deviation in its frequency or in the beating frequency of the oscillator 0 will be instantly indicated by a low frequency beat note. Not only does the intermediate carrier frequency appear at the diagonal of the W'heatstone bridge determined by the points a and b, but the frequencies of the side bands representing the signals will also appear at these points and will pass through the primary winding of T Since this winding also carries currents from oscillator 0 the output of demoduulator G will consist only of the voice frequency components of the signal when the intermediate carrier frequency is adjusted to be the same as the frequency of the oscillator O Thus, the quality of the speech signals may also bemonitored continuously.

While this invention has been shown in certain particular arrangements merely for the purpose of illustration, it W111 be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A. monitoring apparatus for a double detection radio receiver comprising in combination with said radio receiver, means for by-passing part of the energy of the carrier wave at an intermediate frequency, means for producing a wave of substantially the same frequency, and means for obtaining a beat note from said frequencies and for preventing that beat note from being introduced into the circuits of the radiov receiver.

2. The combination of a double detection i-radio receiver, aWheatstone bridge, one diagonal'of which'is' coupled to said radlore- I ceiver for receiving intermediate frequencies therefrom, a source of oscillations. connected to the other diagonal of 'thex Nheatstone bridge, a modulator connected to one of the arms ofthe Wheatstone bridge, and means for detectingthe currents derived from the modulator.

3. A monitoring apparatus for a double detection radio receiver comprising a VVh'eatstone'bridge which 1s balanced, an oscillator connected to one diagonal of the bridge,

means for impressing on the other diagonal 'ed in series with said resistance andsaid circuit as one of "the diagonals of the bridge, a

source of alternating current connected as the other diagonal of the bridge, a modulator coupled to one of the arms of the bridge,

and means for detecting the currents derived from the modulator.

The comb nation of a Ifld10"I'8C6lVeI' having a 'plurallty of circuits, a balanced Zheatstone bridge, a. resistance, a rectifier connected inseries with said resistance as one of the diagonals of the bridge, asource of alternating current connected as the other diagonal of the bridge, means for coupling the series circuit of said rectifier and said resistance with one of the circuits of the radio receiver so that the voltage drop across said resistance will be proportional to thelevel of the'energy flowing through said radio receiver, means for connecting said resistance to another of the circuits of the radio receiver so as to control the gain of the radio receiver in accordance with the voltage drop across said resistance, a modulator coupled to one ofthe arms of the bridge, andmeans for detecting'the currents derived from the modulator. 6. The method ofrmonitoring the carrier curent of a signaling system without interfering with the quality of transmission over the signaling system which consists in bypassing part of the carrier current, and a.

comparing the frequency of the carrier'current with current of a desired, predetermined frequency to determine whether said currents are synchronous. l

7. The method of monitoring the carrier ifz' current of a signaling systemwhich consists in by-passingpart of the energy of the carrier current, modulating the by-passed energy with the energy of an independently produced current of. predetermined fre quency, and preventing any of the products of modulation from interfering With the transmission of signals over the system.

8. Monitoring apparatus for carrier re ceiving circuits comprising in combination with said carrier receiving circuit, means for lay-passing part of the carrier current, means for producing current having substantially the same frequency as that of the carrier current, means for obtaining a beat note from said currents, and means for preventing the introduction of said heat note intov said receivin circuits,

9. Monitoring apparatus for carrier cir cuits comprising in combination With said carrier circuits, means for by-passing part of the energy of the carrier current, means for generating current having a frequency equal to the normal frequency of the carrier current, and modulating means for heating the hy-passed current with the generated current, said modulating means including means to prevent any of the products of modulation from becoming impressed upon said carrier circuits.

10. The method of monitoring a signaling system employing carrier current which consists in lay-passing and separating from the signaling system some of the energy of the carrier current, comparing the frequency of the carrier current With the frequency of an independently generated current to determine whether these currents are of the same frequency, and changing the frequency of the carrier current until the currents compared are of the same frequency.

11. The method of monitoring the circuits of a high frequency signaling system Which consists in by-passing and separating from the signaling system some of the carrier cur-v rent, modulating the by-passed current With other current of predetermined frequency Without introducing any of the products of modulation into the signaling circuits, and changing the frequency of the carrier current so that the difference between the frequency of the carrier current and the other frequency used in modulation is zero.

12. The method of controlling a carrier signaling system which consists in maintain ing the gain of the system constant, simul taneously and conjointly monitoring the carrier energy of the system, and preventing the monitor of the carrier energy of the system from introducing alternating potentials which may interfere With the control of the gain of the system.

1.3. A carrier signaling system comprising means for controlling the gain of the system and maintaining it constant and means directly connected to the gain controlling means for monitoring the frequency of the. carrier energy of the system without introduclng alternating potentials fiovving in the monitoring means into the gain controlling ieans.

14;. In a carrier signalingsystem,'the combination of means for controlling the gain of the system, means directly connected to l the gain controlling means for monitoring the frequency of the carrier current, and means for preventing the monitoring means from introducing alternating potentials which may affect the gain controlling means. In testimony whereof, I have signed my name to this specification this 5th day of December, 1980.

FRED W. SCHRAMM. 

